Decarbonation reactions, metamorphic

M. J. Bickle, The role of metamorphic decarbonation reactions in returning strontium to the silicate sediment mass. Nature 367, pp. 699-704 (1994). 

Where metamorphic petrology is of obvious help is in the area of decarbonation reactions. Although fluid pressures are higher, there are well known reaction sequences. In particular the reaction ... 

Some possible suites of assemblages in the zeolite facies are represented by diagrams a to g in Figure 1. All these suites are represented, for example, in Triassic sediments of the Murihiku Supergroup of New Zealand (cf. 6). The arrows represent dehydration or decarbonation reactions, and hence conventionally increasing metamorphic grade. A possible earlier reaction is ... 

Most metamorphic rocks result from dehydration or decarbonation reactions which involve a fluid phase (heterogenous equilibria). 

Regional metamorphic rocks containing orthopyroxene and K-feldspar i.e., charnockites, must have crystallized in H20-poor environment, or have a significant Fe content. If sufficient H2O were present, such rocks would become either quartz-biotite schists or silicate melts. The low H2O activity can be accoxmted for either by the intrinsic composition of the rocks (metavolcanics) or by the introduction of CO2 into the system by virtue of decarbonation reactions. The fact that many charnockites are fo md in marblebearing terrains is in accord with the requirement for low H2O activity. 

Fig. 85. P-T curves of monovariant equilibria of decarbonation and dehydration reactions in the metamorphism of silicate-carbonate iron-formations containing graphite.

The controls on carbon dioxide would have been somewhat different. Today, carbon dioxide is stored in carbonate minerals in the ocean floor and on the continental shelf. Subduction, followed by volcanism, cycles the carbon dioxide to the mantle and then restores the CO2 to the air. Metamorphic decarbonation of the lower crust also returns carbon dioxide. The carbon dioxide is then cycled back to the water, some via rain, some dissolved via wave bubbles. Erosion provides calcium and magnesium, eventually to precipitate the carbonate. In the earliest Archean, parts of this cycle may have been inefficient. The continental supply of calcium may have been limited however, subseafloor hydrothermal systems would have been vigorous and abundant, exchanging sodium for calcium in spilitization reactions, and hence providing calcium for in situ precipitation in oceanic crust. 

The above discussion suggests that there may be a number of reaction paths by which dolomite and calcite can decarbonate. Campbell has presented the evidence for reactions (1) and (2) being spontaneous at 600°C and at 1 atm pressure (1, 20). Metamorphic studies give arguments for all three of the above reactions having equilibrium constants greater than one at 600°C and 1 atm (15, 19). The conclusion is that, from a thermodynamic viewpoint, decarbonation of oil shale should be complete at 600°C. 

Prograde metamorphism of sediments (and to a lesser degree igneous and metamorphic rocks) causes the liberation of volatile components by the reaction of lower temperature, volatile rich minerals. If no externally derived fluids infiltrate the rock, volatilization is often referred to as closed system even though it is clear that evolved fluids have left the rock. Dehydration is most common, but decarbonation also occurs in carbonate-bearing lithologies (Ferry and Burt 1982) and desulfidation can locally be important (see also Cartwright and Oliver 2000). 

Figure 1. Lowering of 5 0 by batch decarbonation (straight line) and Rayleigh decarbonation (curves). F is the mole fraction of oxygen remaining in the rock. Note that for Rayleigh decarbonation, tends toward -1000%o if all oxygen is volatilized, but that a calc-silicate limit exists such that F > 0.6 for most metamorphic reactions. There is little difference between the results of the batch and Rayleigh models above F = 0.6 (from Valley 1986).

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